AR-15s

Are AR-15 Adjustable Gas Systems A Good Idea?

Odin Blocks

I’ve written a lot on these pages and in my books about ills that can befall AR-15 platform gas systems, and will write more. In a nutshell: excessive pressure getting into the gas system too quickly causes all manner of ill manners.

These include:

  • Excessively quick bolt unlocking
  • Excessively high bolt carrier velocity (back and forth)
  • Excessive hammering of action parts

The “shorter” the guns get, the more these issues arise, and also the greater are their effects.

  • Rifle-length barrel and gas port location: not much really to worry about.
  • Carbine-length: more to worry about.
  • Pistol-length: much to worry about.

The reason is that there’s a whopping lot more gas port pressure nearer the chamber than there is farther down the barrel.

For a reference, “rifle-length” means 20-inch barrel and gas port located approximately 12-inches ahead of the receiver face. Carbine-length is a 16-inch at seven inches ahead. Pistol length may be as short as a 7.5-inch barrel (or even less) with a gas port a scant four inches ahead.

There’s also “mid-length” and “intermediate-length” adjustable gas systems. But more on that in a short bit.

Starting some 20 years ago, we put these on longer-barreled NRA High Power Rifle competition guns, along with relocated (forward) gas ports. Back in that day, these were provided as custom parts by some of the better builders. The idea was to offset the effect of the longer barrel (higher gas pressure in the system) and, largely, to improve spent case condition.

“Adjustable” Gas Blocks

There’s an idea out there running rampant that the best way to fix an excessively gassed AR-15 is to “just slap an adjustable gas block on there” and live happily forever.

Well… about that.

These are indeed worthwhile and effective devices, but only when used for the right reasons for the right circumstances.

An adjustable gas block is best described as a “valved” gas block. The function they provide comes in one of two essential ways: some restrict, some vent.

Those that restrict provide a means to effectively alter the gas port opening size, kind of like opening or closing a water faucet. Those that vent provide an outlet, a bypass, for excessive gas to exit without entering the system—usually out the front of the block itself.

Both of these are user-adjustable. I don’t know that there’s a clearly superior approach, but those that vent tend to hold up a little better.

So, all that sounds like a great idea. If there’s too much gas, make it so there’s not too much gas. And they indeed “work.”

When To Use One

In my experience, which is where I get my opinions, I think an adjustable gas block is a wonderful thing on a rifle-length system. If that rifle is to be used for competitive or otherwise range-only use, I run one.

On my 18-inch barreled, rifle gas port, practical competition rifle, for instance, I use it because that rifle has overall “architecture” that fits into the parameters I have set for an adjustable block—and because it takes every last bit of movement out of the sight.

It’s a worthwhile tradeoff (to me at least) for that. So why not enjoy all that goodness on all of them?

Odin Block - Adjustable Gas Systems
This block from Odin is simple and, so far, has maintained itself well. Technically it’s “tuneable,” not adjustable, and the two large screws tend to stay moveable.

A point that gets missed is that the adjustable gas block becomes a part of the gas system, and it’s the same gas system that was overactive. And it was overactive because it contained just too doggone much high-pressure, really. Hot gas.

And this adjustable gas block is in the same location. The main cause of the excessive gas effect is the gas port location being closer to the chamber.

Adjustable gas blocks won’t last forever, and neither does the adjustment that was once carefully put on it. They get dirty and the heat erodes the innards.

Look in the barrel of a well-worn AR-15 and you’ll see an eroded area just beyond (toward the muzzle) the gas port.

That’s from flame-cutting, the same culprit as what causes chamber throat erosion. The trick is that, after time, the adjustable gas block may not remain adjustable.

Both the erosion and the carbon deposits that get into the valving effectively alter the pressure, getting into and then through the system. This is one area of some difference in the restricting or venting designs, but the result is that function will change.

And there’s a chance nothing can be done about that. I’ve seen them lock, slap-up and no longer adjust.

Superlative Block
Heavy buffers and stouter springs are a longer-term solution for short guns, and they allow pressure to stay “up” enough to reliably cycle.

A Couple Words of Caution

Do not run adjustable gas systems on a pistol-length gun! It will not work well and it will not work long. I also won’t run one on a carbine-length setup for the same reasons.

There’s such a huge dose of hot gas hitting the block interior that it’s not going to hang out over the long haul. And. There has to be enough pressure in the system to cycle reliably.

With the already-abbreviated volume available within the system, there’s less room for error in a short system respecting “enough.” Longer systems are more flexible.

Again, I can recommend adjustable blocks only for use on rifle-length systems. The block is then far enough away that the already-milder dose of excessive gas won’t hurt it… as much.

Two tips on getting the best performance over the longest life with an adjustable block:

  1. Don’t get too greedy on restricting gas flow into the system. Get function 100 percent and then open it up at least a little bit more. This is very important for a gun that may be used in different temperatures and with different brands of ammo.
  2. Shop for a block that can be cleaned and rebuilt, and then clean and rebuild it! I can’t say for certain how many rounds should pass between refurbishings, but I’d sure check it after 1,500.
Adjustable Gas System
On guns this short, adjustable gas systems are not the right answer. They will get eaten up in short order.

Alternatives for Adjustable Gas Systems

I quickly and freely admit that, in some instances, retrofitting an adjustable gas block onto a problematic gun might seem like the most direct and easy solution, and that could be, in fact, right. That is also one of the reasons for their popularity.

However! If you decide that you have to have one to make the gun work, look for solutions elsewhere. There are easier solutions.

Ramp up buffer weight and also buffer spring power, for example. Both delay bolt unlocking to give a little more time for pressures to drop.

Next time around, though, the far and away best solution to reduce the effects of excessive gas port pressure is to locate the gas port farther forward.

I very strongly recommend installing a “mid-length” gas system on any 5.56/.223 carbine (port location +2 inches ahead compared to carbine), and I also recommend moving the port out to carbine location on an AR-platform pistol in the same chambering.

If, and only if, you can live with a long enough barrel to make that possible. An 11.5-inch can work with that location.

Geissele Super 42
Even though they treat the symptoms, not the cause, heavy buffers and stouter springs are a better solution for a shorter gun.

How to Tune an Adjustable Block

Finally, before I end this post, I thought I’d include a section on tuning these adjustable gas systems.

After umpteen years of messing with these things, I suggest closing it down all the way to start. Load only one round at a time (an empty magazine is sitting in the gun, round chambered). Shoot and test. Open the valve apparatus until the bolt locks back.

Then test it with a few magazine-fed rounds. And then open the doggone thing another fourth of a turn! Play it safe. The reason for the empty magazine is because it takes a little more to trip the bolt latch than to feed the next round.

I also, when possible, put a drop of light-duty threadlocker on the adjusting screw. That keeps it in place and reduces the likelihood of corrosion.

Gas Block Adjust
Note: The preceding is a specially-adapted excerpt from Glen’s book, The Practical AR15

Do you think adjustable gas systems are a good idea? Why or why not? Let us know in the comments below

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (17)

  1. Just a quick observation.
    If it is honestly a better solution to move
    the gas port on the barrel for proper tuning,
    why aren’t the barrel manufacturers moving
    The locations or offering this as a solution?

  2. I purchased an AR pistol, 7.5” barrel. It was a reputable brand with a heavy buffer and spring but would never cycle reliably. I installed an adjustable gas block and it functioned perfectly within 15 minutes for $ 50. I could have spend tons of time and money attempting to cycle the pistol with no guarantee it would work. If I have to clean the gas bloc at some point, no problem. But if I needed the pistol to function to save my life, I know it functions reliably with any ammo. Incidentally, I changed the barrel to a 10.5” model as the muzzle blast was ferocious on the shorter barrel. The new barrel has a carbine length system and the gas block is fully opened. A standard bloc will probably function with the carbine length system. It is much more manageable with the longer barrel, reduced muzzle flash and a significant noise reduction. It still is a compact game changer in a difficult situation. 30 round magazine, red dot with 45 degree back up sights. It will easily group 2” at 100 yds.

  3. I have an adjustable gas block on me 300 blackout pistol, it has a pistol length gas port.
    since a black out running subsonic is not reliable on any gas length longer than pistol, including 16″ barrels the adjustable block is not there to restrict the gas, its there to turn it off. when suppressed the 300 blackout is very quiet, and the loudest noise aside from the bullet strike is the action cycling.
    by turning off the gas it is a single shot that is excellent for night predator and varmint control. using a day/night scope I have been able to take several raccoons before they got wise and left the area. and calling in coyotes is easy and the others don’t hear a thing. so my block is basically all the way open or all the way closed, and I have not had any real wear on it as gas pressures are on the low side to start with. since I have never seen extractor marks on my brass even with the hottest supersonic hand loads, I really do think 300 blackout is and should be an exception to your no adjustable blocks on pistol gas systems rule

  4. I have 3 AR type weapons. Two chambered in 5.56 x 45; One 16” carbine & one 10.5” pistol. And a 10.5” chambered in .300 Blackout. I have Anderson adjustable gas blocks on all of them. If you run a sound suppressor on a .300 Blackout pistol you might not get it to work with all ammo without an adjustable gas block. I had to use an adjustable gas block to get mine working with subsonic ammo.

  5. With the exception of different calibers in an AR other than a 5.56 or if not running a suppressor it should not be needed anymore. Most barrel manufacturers these days seem to drill the gas port holes the right size as to not over gas the rifle. It is also how much barrel you have past the port that causes the over pressure. So manly the Carbine length gas tube on a 16” rifle. The 7.5” pistols I have barely eject the brass out and if anything under gassed since only 3.5” of barrel that the bullet can travel to not have all the pressure going out of the front.

  6. Adjustable gas blocks are not for everyone, or every situation
    But depending on the build and the purpose of
    The rifle they can be very useful . I’m building
    A small light weight hiking/ hunting 14.5 pinned build
    Keeping it light and short with a short stock system, with a
    Proprietary spring and buffer weight. Carbine length
    I don’t want over gassing or much muzzle rise.
    This gun will be one that gets dragged around a lot,
    Not shot a lot. For this, a adjustable block opened up a half turn
    over function will be perfect for my needs.
    If you build a rifle to take out and throw hundreds of rounds down range every
    Other weekend an adjustable gas block probably isn’t for you. If you
    Build a gun for a nice scope and want great follow up shots it’s my opinion
    Know how to clean and repair / replace parts . Nothing last forever and even springs wear out
    Specially in over gassed systems.

  7. My .308 Springfield has this. My bolt doesnt stay closed when empty. What do I do? It has a 16 inch barrel with 3 insert vents and wrench.?

  8. I’d certainly not run an adjustable gas block on an AR of any barrel or gas system length that I’d be relying on to defend my life with. I want a simple no frills rifle that goes bang every time I pull on that trigger. No finicky ammo eaters need apply, all of my carbines sport a middy gas setup and they work. However, I do have a 6.5gren with a JP adjustable block on it with a 24″ barrel and rifle gas system.

  9. Why can’t anyone make a tool less adjustable bleed off style gas system for an AR rifle that is similar to that of an FAL?
    Robust, reliable, low maintenance, proven.

  10. … and should have mentioned I have found adjustable gas blocks a necessity with 6.8mm SPC when suppressed (intermediate and longer gas tubes), often only 1 1/2 turns open with the can on. Found I didn’t need an AGB on a 300 BO suppressed with a pistol-length gas tube.

  11. Nice write-up. I use adjustable gas blocks on intermediate and longer gas tubes because I often reload heavy for caliber bullets with slower powders. It might be necessary to state that adjustable gas blocks can only decrease gas flow, they can not increase gas flow if on is undergassed.

  12. Adjustable gas blocks are good if adjusted properly
    when I did my 300blk I had to open the gas port in
    the barrel and go to a weaker recoil spring to get it
    to work.

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